This invention relates to an electromagnetic clutch which couples and uncouples two rotary shafts under an electromagnetic force. It also relates to a rotation transmission device provided with such an electromagnetic clutch for carrying out change over of transmission and shutoff of a driving force in a drive line of various machines and vehicles.
An electromagnetic clutch is used as a device for connecting and disconnecting power in a power transmission line to selectively change over transmission and shutoff of power in various machines and vehicles, such as front and rear wheels of a 4WD car.
FIGS. 9-12 show a conventional rotation transmission device provided with an electromagnetic clutch.
The device has an inner member (inner ring) 1 coupled through a serration 38 to an input shaft 7 so as to rotate together, and has an outer member (outer ring) 2 rotatably mounted concentrically around the inner member 1. A plurality of cam surfaces 9 are formed on the inner member 1, while a cylindrical surface 10 is formed on the outer member 2 to define an annular space between these surfaces. A retainer 5 is provided in the annular space, and rollers 6 performing as engaging elements are retained in a plurality of pockets 8 formed in the retainer 5 to mount the rollers in the annular space. A switch spring 11 is engaged between the retainer 5 and the inner member 1 to bias the retainer 5 to a neutral position where the rollers 6 do not engage the cylindrical surface 10 or the cam surfaces 9.
By changing the phase of the retainer 5, the rollers 6 will engage and disengage between the inner member 1 and the outer member 2. This arrangement forms a clutch.
The electromagnetic clutch portion comprises a rotor 15 having a -shaped section and fixed to a cylindrical rotor support 17 formed of a nonmagnetic material, and comprises an armature 13 provided at one end of the retainer 5 so as to be axially movable but relatively nonrotatable. The rotor is and armature 13 are arranged with a suitable gap kept therebetween by a separating spring 18. An electromagnetic coil 16 for pressing the rotor 15 and the armature 13 against each other by a magnetic force is nonrotatably arranged in the rotor 15 through a field core 3 fixed to the housing 4. By turning on and off a current to the electromagnetic coil 16, the rollers 6 are engaged and disengaged between the inner member 1 and the outer member 2.
When a current to the electromagnetic coil 16 is off, the rollers 6 are biased by the switch spring 11 through the retainer 5 to their neutral position on the cam surfaces 9 formed on the inner member 1, so that the inner member 1 and the outer member 2 can idle.
When it is desired to engage the inner member 1 with the outer member 2, a current is supplied to the electromagnetic coil 16. The armature 13 coupled to the retainer 5 is attracted by magnetic force toward and pressed against the rotor 15.
Frictional torque due to pressing acts between the retainer 5 and the outer member 2 through the armature 13 and the rotor 15. When the inner member 1 and the outer member 2 rotate relative to each other, the frictional torque overcomes the torque of the switch spring 11, so that the retainer 5 and the rollers 6 as the engaging elements rotate with the outer member 2. As a result, the rollers 6 as the engaging elements move from the neutral positions on the cam surfaces to wedge-engagement positions where the inner member 1 and the outer member 2 are engaged together and driven.
The rotor 15 of such an electromagnetic clutch comprises cylindrical portions 15a and 15b formed on the inner-diameter and outer-diameter sides, respectively, and a disk portion 15c that opposes the armature 13 (FIG. 12).
The field core 3 for mounting the electromagnetic coil 16 comprises cylindrical portions 3a and 3b formed on its inner-diameter and outer-diameter sides, respectively, and an end face portion 3c through which it is fixed to the housing 4.
A lead wire 20 for supplying a current to the electromagnetic coil 16 is led out of the field core 3 through the end face portion 3c of the field core 3 out of the housing 4.
As described above, since an electromagnetic clutch is engaged by the attraction force produced by the electromagnetic coil 16, if the attraction force is weak, the armature 13 and the rotor 15 may be kept separate from each other, so that the clutch may not be engaged.
Thus, it is desired to ensure engagement of the clutch by increasing the magnetic attraction force of the electromagnetic coil 16 as much as possible, thereby improving reliability of operation.
But in many applications, the space for installing the electromagnetic clutch portion in the housing 4 is limited, so that the ability to increase the size of the electromagnetic coil 16 is limited.
Thus, a first object of this invention is to increase the magnetic attraction force of the electromagnetic coil as much as possible in a limited installation space, thereby improving reliability of operation.
FIG. 13 shows another conventional rotation transmission device.
In this prior art device, the housing 4 and the outer member 2 are supported through a bearing 33, and entry of foreign matter into the rotation transmission device is prevented by sealing devices 50, 51 provided at both ends of the housing 4.
As the sealing device 50, which is provided on the open side of the housing, a combination of a sealing plate such as a slinger and a sealing lip is used. The sealing device 50 is pressed into the end face of the housing 4 on its open side.
Thus, if during operation the housing 4 and the outer member 2 are displaced relative to each other in the axial direction due to rattling, and the sealing plate of the sealing device 50 protrudes outside through the opening of the housing 4, the sealing properties may deteriorate.
Under use conditions where there are many flying stones, if the sealing device 50 is located at the end face of the housing 4 on its open side, a flying stone may hit, for example the sealing plate of the sealing device 50, thus deforming it and deteriorating the sealing property.
Also, in this rotation transmission device, a bearing 33, which supports the outer member 2 and the housing 4, is provided on the outer peripheral side of the portion forming the clutch. The bearing 33 comprises an inner ring 33a, an outer ring 33b and a plurality of rolling elements 33c. When the clutch engages, the rollers 6 engage the outer member 2 and the inner member 1, so that the outer diameter of the outer member 2, which forms the clutch, increases. This increases the load acting between the inner ring 33a and the rolling elements 33c of the bearing 33, so that if the interference between the outer ring 2 and the bearing 33 is large, the bearing 33 may be damaged.
Also, in conventional rotation transmission devices, grease is sealed by providing sealing members 33d on both sides of the inner ring 33a and the outer ring 33b of the bearing 33. However, separately carrying out lubrication of the bearing 33 and lubrication of the clutch portion causes costs to increase.
Thus, a second object of this invention is to prevent deterioration of sealing, damage to the bearing and cost increase in the rotation transmission device.
According to this invention, in order to solve the first object, the dimensions of the field core are optimized to increase the magnetic attraction force of the electromagnetic coil as much as possible.
Heretofore, the field core 3 was formed by press molding, and as shown in the enlarged view of FIG. 12, the thickness A of the cylindrical portion 3a on the inner-diameter side of the field core 3 had the same thickness as the thickness B of the cylindrical portion 3b on the outer-diameter side. Thus, if the sectional area of the cylindrical portion 3a on the inner-diameter side is compared with that of the cylindrical portion 3b on the outer-diameter side, the sectional area of the cylindrical portion 3a on the inner-diameter side is smaller, so that magnetic saturation tends to occur in only part of the cylindrical portion 3a on the inner-diameter side. Magnetic efficiency is thus bad.
According to this invention, the thickness A of the cylindrical portion 3a on the inner-diameter side of the field core 3 is larger than the thickness B of the cylindrical portion 3b on the outer-diameter side. In other words, the relation A greater than B is met so that the sectional areas of both cylindrical portions 3a, 3b are equal to each other, and thus the magnetic flux density distribution is uniform. By making the magnetic flux density distribution uniform, a magnetic circuit is formed efficiently, so that a large electromagnetic force is obtained in a limited installation space.
Similarly, in this invention, the dimensions of the rotor are also optimized to increase the magnetic attraction force of the electromagnetic coil as much as possible. Heretofore, as shown in FIG. 12, in the rotor 15 too, the thickness C of the cylindrical portion 15a on the inner-diameter side had the same thickness as the thickness D of the cylindrical portion 15b on the outer-diameter side. That is, they meet the relation C=D. Thus, the sectional area of the cylindrical portion 15a on the inner-diameter side is smaller than that of the cylindrical portion 15b on the outer-diameter side, so that magnetic saturation tends to occur only in part of the cylindrical portion 15a on the inner-diameter side, thus worsening magnetic efficiency.
According to this invention, the thickness C of the cylindrical portion 15a on the inner-diameter side is larger than the thickness D of the cylindrical portion 15b on the outer-diameter side, i.e. the relation C greater than D is met so that the sectional areas of cylindrical portions 15a and 15b are equal to each other, thus making the magnetic flux density distribution uniform.
Further, according to this invention, the wall thickness of the end face portion 3c forming the field core 3 is equal to or larger than that of the cylindrical portion 3a on the inner-diameter side to prevent magnetic saturation at the inner-diameter portion of the end face portion 3c and make the magnetic flux density distribution uniform.
Also, according to this invention, in order that mis-engagement will not occur at the electromagnetic clutch portion due to viscous resistance of grease that stays between the rotor 15 and the armature 13, grease discharge holes 21 are formed in the outer periphery of the attraction surfaces of the rotor 15 and the armature 13.
Further, heretofore, the lead wire that supplies current to the electromagnetic coil 16 was pulled out of the field core 3 through the end face portion 3c of the field core 3 and was directly pulled out of the housing 4 to outside. Thus, if the lead wire 20 is pulled by accident, the mounting portion of the lead wire to the field core 3 may be broken.
In view of this possibility, according to this invention, a lead wire 20 that has been led out into the housing 4 through the end face portion 3c of the field core 3 is wound on a cylindrical portion in the housing and its end is pulled out of the housing so that the tension of the lead wire 20 will not directly act on the mounting portion of the lead wire 20 to the field core 3.
Also, according to this invention, in order to solve the second object, a sealing device for preventing entry of foreign matter into the housing is provided between the housing and at least one of the inner member and the outer member, and the sealing device is recessed from an end face of the housing, thereby preventing deterioration of sealability.
By recessing the sealing device from the end face of the housing, even if the housing and the outer member become displaced relative to each other due, for example, to axial rattling, it is possible to prevent the seal plate of the sealing device from protruding to the outside through the opening of the housing.
Also, by recessing the sealing device from the end face of the housing, flying stones are less likely to hit the sealing device, so that it is possible to lessen damage to the sealing device itself.
Further, by providing a shield plate outside of the sealing device, it is possible to protect the sealing device against foreign matter such as flying stones, mud and grass.
Also, the sealing device may comprise two annular sealing plates each having an L-shaped section, having a cylindrical portion, and having an upright plate portion, and a sealing lip.
On the open side of the housing, a bearing sealing member for sealing the bearing may be provided and a sealing member on the opposite side of the housing may be omitted. This reduces the cost of the bearing sealing member itself. Also, due to the fact that the bearing portion and the clutch forming portion communicate with each other, grease for the bearing portion and grease for the clutch forming portion can be used in common.
Also, if the bearing is formed of an outer ring, an inner ring and a plurality of spherical rolling elements, and if the bearing is arranged on the outer-diameter side of the outer member as the clutch forming portion, by setting the interference between the bearing and the outer member such that when a predetermined torque is loaded on the clutch, the contact stress at the center of the contact points between the inner ring of the bearing and the rolling elements will be 4200 MPa or below, and it is possible to prevent damage to the bearing.
In the rotation transmission device of this invention, either rollers or sprags may be used as the engaging elements.